I. Field
The following description relates generally to communication systems, and more particularly to a method and apparatus for soft symbol determination.
II. Background
Multimedia Broadcast and Multicast Services (MBMS) is a broadcasting service offered via existing cellular network technologies, including Global System for Mobile Communications (GSM), and Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP) and successor to GSM. The infrastructure offers an option to use an uplink channel for interaction between the service and the user, which is not a straightforward issue in usual broadcast networks, as for example conventional digital television is only a one-way (unidirectional) system. MBMS uses multicast distribution in the core network, instead of point-to-point links for each end device.
In MBMS, there may be more than one base station providing the same service, and their signals are often combined to more reliably recover the broadcasted information. There are two types of combining that are possible pursuant to the 3GPP standard. The first, referred to as soft combining, occurs at the physical layer, signaling level. The gains achieved from this approach are higher than the second approach. However, this approach also requires accurate knowledge of the delays between various Base Stations. Incorrect delay information may severely degrade the performance to the extent that soft combining will provide more negative results over no combining.
In addition to the soft combining approach, a second approach of signal combination, referred to as selection combining, happens at the Radio Link Control (RLC) layer level. This combining approach does not require delay information about the base stations. Hence, it is immune to in-accurate delay information. However, the gains achieved from this type of combining are not as significant as compared to soft combining.
Although soft combining provides better results than selection combining, effective soft combining poses several challenges, including: ensuring robustness to network synchronization issues; optimal data path combining in the absence of dedicated pilot bits; and optimal data transport format detection over multiple links each possibly using a different transport format.
Consequently, it would be desirable to address some of the issues noted above.